Bleaching process

ABSTRACT

IN THE PROCESS FOR BLEACHING A MINERAL WHEREIN THERE IS FORMED A FLOWING STREAM OF AN AQUEOUS SUSPENSION OF THE MINERAL AND WHEREIN A DITHIONITE BLEACHING COMPOUND IS DISPERSED IN SAID STREAM TO BLEACH SAID MINERAL, THE IMPROVEMENT WHICH COMPRISES DISPERSING SAID DITHIONITE BLEACHING COMPOUND IN SAID STREAM AT A KNOWN RATE, PASSING A QUANITY OF SAID STREAM AT A STEADY RATE OF FLOW AND AFTER THE DITHIONITE HAS BEEN DISPERSED IN SAID STREAM FOR A TIME RANGING FROM 1/4 MINUTE TO 30 MINUTES THROUGH A HOUSING CONTAINING AN ELECTRODE ASSEMBLY FOR A POLAROGRAPH, MEASURING THE POLAROGRAPHIC DIFFUSION CURRENT CORRESPONDING TO THE CONCENTRATION OF UNREACTED DITHIONITE BLEACHING COMPOUND REMAINING IN SAID STREAM, AND IF NECESSARY ADJUSTING THE RATE AT WHICH THE DITHIONITE BLEACHING COMPOUND IS DISPERSION IN SAID STREAM TO ENSURE THAT THE CONCENTRATION OF THE DITHIONITE BLEACHING COMPOUND IN SAID STREAM IS AT A DESIRED LEVEL.

April 6, 1971 I J B, Rows E'I'AL I 3,573,943

BLEACHING rnocEss Filed Aug. 5, 196a 2 Sheets-Sheet z FIG. 3.

. f I (34-00 H e 0 United States Patent US. Cl. 106288 7 Claims ABSTRACT OF THE DISCLOSURE In a process for bleaching a mineral wherein there is formed a flowing stream of an aqueous suspension of the mineral and wherein a dithionite bleaching compound is dispersed in said stream to bleach said mineral, the improvement which comprises dispersing said dithionite bleaching compound in said stream at a known rate, passing a quantity of said stream at a steady rate of flow and after the dithionite has been dispersed in said stream for a time ranging from A minute to 30 minutes through a housing containing an electrode assembly for a polarograph, measuring the polarographic diffusion current corresponding to the concentration of unreacted dithionite bleaching compound remaining in said stream, and if necessary adjusting the rate at which the dithionite bleaching compound is dispersed in said stream to ensure that the concentration of the dithionite bleaching compound in said stream is at a desired level.

BACKGROUND OF THE INVENTION This invention relates to the bleaching of minerals with dithionite bleaching compounds and, more particularly but not exclusively, is concerned with the bleaching of a clay mineral with dithionite solutions.

Solutions of sodium or zinc dithionite are used in the bleaching of minerals, e.g. clay minerals, barytes, strontianite and tale, in which impurities present in the mineral are chemically reduced. Advantageously, the bleaching of a mineral with dithionite is carried out by a substantially continuous process in which the dithionite is dispersed in a flowing stream containing the mineral to be treated. The amount of dithionite bleaching compound required depends on the amount of impurities present in a mineral and in the continuous process the amount of impurities present may vary significantly during the process. In order to allow for this possibility, it is possible to add at all times a quantity of dithionite bleaching compound in excess of the required amount. However, this is clearly wasteful and it is desirable to be able to control the addition of the dithionite to the stream in such a way that the use of quantities of dithionite in excess of that required for a desired bleaching effect is avoided.

It is therefore an object of the present invention to provide a process for bleaching a mineral with a dithionite bleaching compound in which the addition of the dithionite bleaching compound to the mineral is controlled.

SUMMARY OF THE INVENTION Accordingly the present invention provides, in a process for bleaching a mineral wherein there is formed a flowing stream of an aqueous suspension of the mineral and wherein a dithionite bleaching compound is dispersed in said stream to bleach said mineral, the improvement which comprises dispersing said dithionite bleaching compolarographic difiusion current corresponding to the conof said stream at a steady rate of flow and after the dithionite has been dispersed in said stream for a time ranging ice from A; minute to 30 minutes through a housing containing an electrode assembly for a polarograph, measuring the polarographic dilfusion current corresponding to the concentration of unreacted dithionite bleaching compound remaining in said stream, and if necessary adjusting the rate at which the dithionite bleaching compound is dispersed in said stream to ensure that the concentration of the dithionite bleaching compound in said stream is at a desired level.

The concentration of unreacted dithionite bleaching compound remaining in the stream after A minute is very low when the rate of addition of the dithionite bleaching compound is below that required for the maximum bleaching effect, but the concentration of the unreacted dithionite bleaching compound in the stream rises rapidly as the region of maximum bleaching eifect is reached. It may be noted that sulphides and thiosulphates are formed as by-products of the bleaching process and these materials also give a polarographic diffusion current which is indistinguishable from that given by unreacted dithionite. However, the proportions of these by-products present in the stream are small, especially at points not more than 20 minutes downstream of the point at which the dithionite is added to the stream, and do not cause difl'lculty in operating the process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It is found that if the concentration of solids, e.g. clay mineral, in the stream is very high, there is a tendency for stagnant areas to form in the stream. If the diffusion current is to be proportional to the concentration of dithionite present, there must be a steady stream passing the electrodes of the polarograph. In the case of a clay mineral, it is'preferable, therefore, if the concentration of solids in the stream is adjusted to lie in the range 7.5% to 35% by weight. In addition, it has been found that best results are obtained if the velocity of the stream through the electrode housing is just sufiicient to cause the mercury drops to fall from the end of the capillary sooner than they would in a stagnant suspension. Preferably, the stream containing the mineral and dithionite is passed through the housing at a steady rate of flow in the range of from 0.75 to 15.0 litres/minute.

If the conditions obtaining in the main stream are such that the requirements noted above are not readily fulfilled, it is advantageous to tap off a quantity of the main stream and measure the concentration of dithionite in this quantity. This can be done by collecting in a funnel a quantity of the main stream as it flows over a weir, passing the collected quantity through the housing and finally returning it to the main stream.

Preferably, the, quantity of the stream which is passed through the housing is passed therethrough after the dithionite has been dispersed in the stream for at least one minute and not more than 10 minutes.

The amount of dithionite bleaching compound dispersed in the stream containing the mineral varies considerably according to the mineral concerned and its origin. In the case of clay minerals, the amount of dithionite bleaching compound required will normally lie in the range 0.2 to 10.0 lbs. per ton (long ton) of dry clay mineral.

Preferably, the electrode assembly comprises a dropping mercury electrode and a calomel electrode.

Two embodiments of a polarograph suitable for use in the process of the persent invention are shown, by way of example, in FIGS. land 2 of the accompanying drawings.

Referring first to FIG. 1, the polarograph comprises an electrode assembly, including a saturated calomel electrode 1 and a dropping mercury electrode 2, disposed in a cylindrical housing 6, the housing being provided with a number of apertures 10 so that the stream containing the mineral being bleached by the dithionite bleaching compound can pass through the housing. Both electrodes are supported by a central tubular column within which there are disposed supply tubes 8 and 9 for the two electrodes. One supply tube *8 extends from a mercury reservoir 4 and the other supply tube 9 extends from a reservoir of saturated potassium chloride solution. A lead 11, for the calomel electrode, passes through the supporting column 7 whilst electrical connection to the mercury electrode is by means of a platinum wire 12, or a stainless steel rod, immersed in the mercury reservoir. Mercury falling from the capillary of the dropping mercury electrode is trapped in a well 3, which is shaped like an inkwell, from where it may be drained periodically on removal of a plug 13. In operation, the housing 6 containing the two electrodes is immersed in the stream containing the mineral being bleached by the dithionite bleaching compound and the diffusion current corresponding to the dithionite concentration measured.

[If it is necessary to tap off a quantity of the main stream and to measure the dithionite concentration in this quantity, it is advantageous to modify the housing of the polarograph shown in FIG. 1. A suitably modified housing is shown in FIG. 2 of the accompanying drawings; the housing 6 is provided with an inlet tube 20 and overflow holes 20a. As with the embodiment shown in FIG. 1, the housing 6 contains an electrode assembly which essentially comprises a calomel electrode 1 and a dropping mercury electrode 2. The mercury drops are collected in a well 3.

The theoretical relationship between diffusion current and dithionite concentration is of the form:

in which I is the average current during the life of a drop, K is a constant, m is the mass flow rate of mercury for the dropping electrode, t is the drop time, and C is the concentration of dithionite. The size of the mercury drop depends only on the internal diameter of the capillary tube in the electrode assembly and, as different electrode capillary tubes have slightly different diameters, provision is preferably made to adjust the hydraulic head on the mercury flowing through the capillary, so that the drop time (i.e. the time for one drop to grow and detach itself) and the mass flow rate can be kept constant. In this way, i.e. by varying the mercury drop size and mercury flow rate, the electrode unit can be adjusted to give the same diffusion current for a given dithionite concentration, this continuously variable sensitivity being useful as it enables the dithionite concentration to be read-off directly.

The diffusion current, which is of the order of 3 microamps, is proportional to the concentration of dithionite present in the stream passing the electrodes and can be amplified for recording or controlling purposes. One form of amplifier-recorder consists of a high gain DC amplifier provided with a feed-back loop to control the overall gain, the input signal being the potential difference between the ends of a 500 ohm resistor in the electrode circuit; the amplifier output is then applied to a pen recorder. Alternatively, there may 'be used a commercial amplifierrecorder with a full scale deflection for a one millivolt input; a 500 ohm potentiometer is connected in the electrode circuit and provides a voltage proportional to the diffusion current, of which a proportion is applied to the input of the amplifier-recorder depending on the overall sensitivity required. The amplifier-recorder can be made to give a direct reading of the dithionite concentration by adjusting the 500 ohm potentiometer until a given known dithionite concentration corresponds to an appropriate scale deflection. In this way variations in the diameter of the bore of the capillary tube can be allowed for. It will be appreciated that the diffusion current varies as each mercury drop grows and then detaches itself from the capillary; and in order to damp out the fluctuation in current, a bank of capacitors can be connected in the input circuit of the amplifier to give a time constant of 10 to 4 seconds. -If an indication of the order of concentration of the dithionite in the stream passing the electrodes only is required, the electrode'.unit may be connected directly to a low resistance microam-meter, no external power supply being required.

The invention is illustrated by the following example.

EXAMPLE A paper coating clay having an initial reflectance to standard violet light (458 m of 84.2% and a particle size distribution such that 79% by weight consisted of particles smaller than 2 microns was formed into a suspension having a solids content of 15% by weight and bleached using a 15% by weight solution of zinc dithionite in water.

In 'order to measure the concentration of dithionite in the suspension after the bleaching reaction had neared completion there was employed a polarographic apparatus as described with reference to FIG. 2 of the accompanying drawings. The apparatus was calibrated, by adjusting the height of the mercury reservoir above the capillary until the drop time was four seconds, so that a diffusion current of l microamp represented 16:2 parts per million by weight of dithionite. A quantity of the main stream of the suspension, in which there was dispersed a quantity of the dithionite solution, was tapped ofi from the main stream of the suspension at a point such that there was a time lag of 1 /2 minutes between the point at which the dithionite solution was added to the main stream and the point at which the quantity was passed through the apparatus. The test quantity was tapped off from the main stream and passed through the electrode unit at a flow rate of 4.9 liters per minute. It may be noted here that, with the apparatus shown in FIG. 2, a volume flow rate of 5 liters/minute corresponds to a linear flow rate in the inlet tube 20 of 1%. feet per second.

The rate of addition of the dithionite solution to the clay suspension was measured with a Rotameter, and from the Rotameter readings and the mass flow rate of the clay through the bleaching plant, the zinc dithionite dose rate, in pounds of dithionite per ton of dry clay, could be calculated. The rate of addition of the dithionite solution was adjusted to give a range of dose rates from 0.5 to 5.7 lbs. per ton of dry clay. At each dose rate the diffusion current was measured and at the same time a sample was withdrawn from the main stream at the same point as the quantity to be passed through the polarograph was tapped off, and evaporated to dryness under standard conditions of time and temperature. The reflectance to violet light of the dry clay was then measured. The results are shown graphically in FIG. 3 of the accompanying drawings in which curve I shows the brightness of the clay as a function of the zinc dithionite dose rate and curve II shows the ditfusion current as a function of the zinc dithionite dose rate. It can be seen (a) that there is little or no increase in brightness when the dose rate is increased beyond 3.5 lbs. of dithionite per ton of dry clay and (b) that for dose rates in excess of that required to produce maximum brightness the increase in diffusion current is directly proportional to the excess dose rate.

The process of the invention can be used for minerals other than clay minerals. For example, the process of the invention can be used in the bleaching of barytes (barium sulphate), strontianite (strontium carbonate) and talc.

What is claimed is:

1. In a process for bleaching a mineral wherein there is formed a flowing stream of an aqueous suspension of the mineral and wherein a dithionite bleaching compound is dispersed in said stream to bleach said mineral, the improvement which comprises dispersing said dithionite bleaching compound in said stream at a known rate, and, after the dithionite has been dispersed in said stream for a time ranging from A minute to 30 minutes, passing a.

quantity of said stream at a steady rate of flow through a housing containing an electrode assembly for a polaro= graph, measuring the polarographic diffusion current cor= responding to the concentration of unreacted dithionite bleaching compound remaining in said stream, and if necessary adjusting the rate at which the dithionite bleaching compound is dispersed in said stream to ensure that the concentration of the dithionite bleaching compound in said stream is at a desired level.

2. A process according to claim 1, wherein said mineral is a clay mineral.

3. A process according to claim 2, wherein said quantity of the stream is passed through the housing containing an electrode assembly for a polarograph at a steady rate of flow in the range of from 0.75 to 15.0 liters/ minute.

4. A process according to claim 2, wherein said aqueous suspension of the mineral has a solids content in the range of from 7.5% to 35% by weight.

5. A process according to claim 1, wherein said quantity of the stream is passed through the housing containing an electrode assembly for a polarograph not more than 10 minutes after the dithionite bleaching compound is dispersed in said stream.

6. A process according to claim 3, wherein said aque ous suspension of the mineral has a solids content in the range of from 7.5% to 35 by weight.

7. A process according to claim 6, wherein said quan= tity of the stream is passed through the housing containing an electrode assembly for a polarograph not more than 10 minutes after the dithionite bleaching; compound is dispersed in said stream.

References Cited UNITED STATES PATENTS 3,399,960 9/1968 Conleyetal.mi im...106-72 JAMES E. POER, Primary Examiner US. Cl. XR. 

